In prior art techniques, a polymetal gate structure has been used for lowering electrical resistance of a gate electrode, wherein a gate electrode structure comprises laminating a metal nitride film and a refractory metal film as a reaction protection film on polycrystalline silicon (for example, refer to patent documents 1, 2, 3, 4, and 5).
Moreover, in a layer structure of polycrystalline silicon/refraction protection film, which is metal nitride film/refractory metal layer, there is one example which describes a structure, wherein a metal silicide is inserted between a refraction protection layer and polycrystalline silicon because the contact resistance of a gate electrode becomes greater (for example, refer to patent document 6).
Furthermore, in a polymetal gate structure, one document describes an oxidation method to prevent a gate electrode from oxidation of the refractory metal and diffusion of boron in the polycrystalline silicon (for example, refer to patent document 7).
Moreover, in a dual gate electrode structure comprising p-type polycrystalline silicon and n-type polycrystalline silicon, especially in the case of a polycide gate structure comprising a laminate structure of tungsten silicide (a silicon compound of tungsten) on a p-type polycrystalline silicon and n-type polycrystalline silicon, boron existing in a p-type silicon region and phosphorus existing in a n-type silicon region diffuse mutually through a tungsten silicon compound (tungsten silicide) film formed on a polycrystalline silicon (for example, refer to non-patent document 1). One document describes a structure isolating a tungsten silicide layer to inhibit mutual-diffusion of impurities doped in the above-mentioned p-type polycrystalline silicon and n-type polycrystalline silicon (for example, refer to patent documents 8, 9, and 10). However, no document describes experimental data about mutual-diffusion of similar impurities in a polymetal gate structure without tungsten silicide and only small descriptions can be found in the above-mentioned Official Gazette.
One document describes a structure, wherein a second barrier metal film remains connected and a part of tungsten silicide of a first barrier metal film is isolated, to avoid the possibility that doped impurities diffuse mutually into polycrystalline silicon films of p-type regions and n-type regions through the tungsten silicide film similar to the above-mentioned polycide gate, in a polymetal gate comprising a laminate structure of a first barrier metal film; tungsten silicide/second barrier metal film; tungsten silicide/metal layer on a polycrystalline silicon film (for example, refer to non-patent document 11).
[Patent document 1] JP-A No. 123060/1985 (p2, FIG. 2)[Patent document 2] JP-A No. 152076/1986 (p2, FIG. 1)[Patent document 3] JP-A No. 119763/1991 (p3, FIG. 1)[Patent document 4] JP-A No. 94716/1995 (p3, FIG. 1)[Patent document 5] JP-A No. 264531/1996 (p3, FIG. 2)[Patent document 6] JP-A No. 298186/2001 (3, FIG. 1)[Patent document 7] JP-A No. 330468/1999 (p5, FIG. 11)[Patent document 8] JP-A No. 5603/1994 (p3, FIG. 1)[Patent document 9] JP-A No. 181194/1997 (p4, FIG. 1)[Patent document 10] JP-A No. 289257/1997 (p5, FIG. 9)[Patent document 11] JP-A No. 217311/2002 (p4, FIG. 2)[Non-Patent Document]
0.25 μm W-Polycide Dual Gate and Buried Metal on Diffusion Layer (BMD) Technology for DRAM-Embedded Logic Devices, 1997 Symposium on VLSI Technology, Digest of Technical Papers pp.23-24, M. Tsukamoto, H. Kuroda, Y. Okamoto
In a CMOSLSI, wherein a circuit comprises a fine MOSFET with a gate length of 0.18 μm or less, a DRAM and a memory-logic embedded DRAM, wherein a similar gate layer is used for the gate electrode and the circuit, a method of manufacturing a gate using low-resistance electrically conductive materials including a metal layer is thought to be employed to decrease gate delay and signal delay in the circuit and to ensure high-speed operation.
A most promising candidate for the low electrical resistance gate electrode material is a so-called polymetal comprising a laminated refractory metal film on a polycrystalline silicon film. A polymetal may be used not only for a gate electrode material but also for a wiring material because the sheet electrical resistance is as low as about 2Ω/□. As a refractory metal, W (tungsten) and Mo (molybdenum), etc. are used, which exhibit excellent low-resistance properties even in a low-temperature process below 800° C. as well as excellent electro-migration tolerance. Moreover, a practical polymetal gate comprises a trilayer structure, wherein a barrier layer consisting of metal nitride film such as WNx (tungsten nitride) is inserted between a polycrystalline silicon film and a refractory metal film, because the adhesive strength between the two films decreases or a high-resistance silicide layer is formed at the boundary of the two films during a high-temperature heat-treatment process when these refractory film is laminated directly on a polycrystalline silicon film. In this structure, because a WNx film is thought to have the ability to protect against diffusion of such impurities as phosphorus and boron in a polycrystalline silicon film into a metallic layer, it has been thought that the effects of multi-diffusion of impurities are not so significant between an n-channel region and a p-channel region through a metallic layer such as polycide gate.
However, from the results of evaluating mutual diffusion of impurities by a heat-treatment process using a sample comprising a trilayer structure, wherein a barrier layer of a metal nitride film such as a WNx (tungsten nitride) film was inserted between a polycrystalline silicon film and a refractory metal film, the inventors confirmed that phosphorus and boron of n-type and p-type polycrystalline silicon diffused mutually through the WNx film and the tungsten film formed thereon and that the ease of diffusion was similar to that of a polycide gate. In addition, experiments carried out by the inventors made it clear that the contact resistance between the polycrystalline silicon and these metals or metallic compounds increased due to diffusion of the impurities.